The present invention relates to optical switches for fiber optic systems, and in particular to a two-input, two-output device for controllably coupling light from a first input optical fiber to either of two output optical fibers.
Mach-Zender type interferometers are often used to construct optical devices. In a Mach-Zender interferometer, input light is divided into two light beams. The beams travel along two parallel intermediate paths and are then recombined and allowed to interfere. A switch based on this interferometer has two output paths. The interference of the light from each of the two intermediate paths determines which output path the light takes.
One such switch has two inputs and two outputs. Light from the first input is divided into two portions, half taking the first intermediate path and half taking the second intermediate path. When the two intermediate paths rejoin, the light from the first input interferes constructively at one of the outputs and destructively at the other output. The light from the first input is thereby coupled to only one of the outputs. Typically light from the second input is similarly coupled to the other output.
To operate the switch, the output that receives the light from the first input is selected by controllably changing the difference between the optical path lengths of the two intermediate paths, thereby changing the interference conditions at the two outputs. In an xe2x80x9coffxe2x80x9d position, light entering the switch through the first and second inputs is coupled to the first and second outputs, respectively. In the xe2x80x9conxe2x80x9d position, light from the first and second inputs is coupled to the second and first outputs, respectively.
In a number of prior art optical switches, the intermediate paths exist within a bulk material. A switch having fiber optics coupled to a bulk material has the following disadvantages: some light is lost in the coupling between the different materials, the switch is sensitive to temperature changes due to the unequal thermal properties of the dissimilar materials, and the switch is bulky and expensive. For use in fiber optic systems, therefore, it is desirable to make the switch entirely of fiber optic components.
U.S. Pat. No. 5,383,048 by Seaver and U.S. Pat. No. 4,772,083 by Ahmed show optical systems using a Mach-Zender type interferometer. However, these devices do not have two inputs and two outputs, and are not made entirely of fiber optic components.
U.S. Pat. No. 5,502,781 by Li et al. shows a two-input, two-output switch, but the switch is not constructed entirely of fiber optic components.
In U.S. Pat. No. 4,929,830, Bohnert et al. use a Mach-Zender interferometer as an electric field sensor. The device does not have two inputs and two outputs, is not made entirely of optical fibers, and is not used as a switch. U.S. Pat. No. 4,477,723 by Carome et al. also discloses an electric field sensor using a Mach-Zender interferometer. This sensor has two inputs and two outputs, and is made of fiber optic components; however, the device is not used as a switch.
U.S. Pat. No. 4,560,234 by Shaw et al. discloses a two-input, two-output fiber optic switch based on a Mach-Zender interferometer. The optical fiber that defines the first intermediate path is wrapped around a cylindrical piezo-electric crystal. The crystal is used to stretch the fiber, thereby changing its optical path length with respect to the second intermediate path, whose length remains stationary.
The switch of Shaw et al. also includes a polarization controller in the first intermediate path to ensure that the polarization of light traversing the first intermediate path remains parallel to the polarization of light taking the second intermediate path. Such polarization control is needed because the interference conditions that allow the switch to function require that the light from the two intermediate paths have the same polarization. As is well known, two light beams will not interfere if their polarizations are perpendicular.
The switch of Shaw et al., however, suffers from the following two disadvantages. First, the inclusion of the polarization controller makes the switch excessively bulky. Second, the piezo-electric device deposits heat into the first intermediate fiber, thereby changing its optical path length. The interference relationship between the two intermediate paths therefore changes with time, resulting in a change in the voltages that need to be supplied to the piezo-electric crystal to achieve the xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d positions. An elaborate feedback mechanism must therefore be connected to the switch outputs and to the piezo-electric crystal to determine the required voltages.
It is therefore a primary object of the present invention to provide a two input, two output fiber optic switch that does not require a polarization controller. It is another object to provide a switch having the property that the voltages used to turn the switch xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d are approximately constant in time.
The invention has the advantage that it provides a compact and inexpensive fiber optic switch. The invention has the further advantage that it has a simple design, since no complicated feedback mechanism needs to be used to monitor the voltages necessary to reach the xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d positions.
An optical switch having fiber optic components is described. The switch comprises first and second input optical fibers, first and second intermediate optical fiber segments, and first and second output optical fibers. The input optical fibers are coupled to the intermediate optical fiber segments by a first optical coupler. The intermediate optical fiber segments are also coupled to the output optical fibers by a second optical coupler. The switch therefore comprises a Mach-Zender type interferometer.
The first and second intermediate optical fiber segments have first and second optical path lengths, respectively. The difference, xcex94l, between the first and second optical path lengths is controllably altered. In the preferred embodiment, first and second piezo-electric plates are attached to the first and second intermediate fiber segments, respectively. The piezo-electrical plates are used to alter the optical path length difference xcex94l.
Light having a vacuum wavelength xcex enters the switch through at least one of the input fibers. The light is divided between the intermediate fiber segments by the first optical coupler. The light traveling through the first intermediate fiber segment then interferes with the light traveling through the second intermediate fiber segment within the second optical coupler. The optical path length difference xcex94l is controllably altered to selectively couple the first and second input fibers either to the first and second output fibers, respectively, or to the second and first output fibers, respectively.
In the preferred embodiment, the optical path length difference xcex94l is altered by increasing the optical path length of the first intermediate fiber segment by an amount d1 and decreasing the optical path length of the second intermediate fiber segment by an amount d2. It is preferred that d1=d2=d, so that equal amounts of heat are deposited in the first and second intermediate fiber segments. In this embodiment the change in xcex94l needed to flip the switch from an xe2x80x9coffxe2x80x9d position to an xe2x80x9conxe2x80x9d position, or from the xe2x80x9conxe2x80x9d position to the xe2x80x9coffxe2x80x9d position, does not change in time since the operation of the switch warms both of the intermediate fiber segments equally.
In the preferred embodiment, the first and second intermediate fiber segments lie substantially in a plane. Because of the planar geometry, the polarization of the light traveling through the first intermediate fiber segment does not rotate significantly with respect to the polarization of the light traveling in the second intermediate fiber segment. Therefore the interference that causes the switch to function is maintained, and no polarization controller is necessary. In one embodiment, the relative rotation between the polarizations of the light traveling in the first and second intermediate fiber segments, respectively, is further reduced by annealing the intermediate fiber segments after assembly.
In one embodiment, a pressure device is provided to apply pressure to one of the intermediate fiber segments, thereby producing a change in the optical path length of the intermediate fiber segment due to the transverse photoelastic effect. An offset in xcex94l is therefore created, so that the switch maintains a desired default position when no voltage is supplied to the piezo-electric plates.